Chapter 8: Combining and choosing analytical techniques
Mass Spectroscopy
Mass Spectroscopy is commonly combined with other analytical techniques. It can
measure both quantitatively and qualitatively. It is mostly used to analyse organic
compounds. Applications include: analysis of pharmaceutical samples, medical
screening, contaminants in water, geological samples of oil and gas.
Mass Spectrometer – how it works!
• Vaporisation: Sample must enter as a gas
• Ionisation: Atoms of the gaseous sample are bombarded with electrons; mainly
singly charged positive ions are formed.
• Acceleration: The ions are accelerated by a strong electrical field.
• Deflection: The ions are deflected in circular paths, by powerful magnetic fields,
according to their mass and their charge- the greater the deflection, the lower the
mass. (m/e)
• Detection: The intensities of different ion beams are detected electronically.
• Collection: The collector records the data (current) as a mass spectrum which is
a graph of percentage relative abundance against relative isotopic mass. Only
particles of a particular radius will reach the detector. The strength of the
magnetic field is altered for other particles to reach the detector.
• Calibration: The instrument is calibrated against a standard isotope (carbon-12)
which is given a value of 12 units exactly.
How the spectrum is formed
A range of peaks are produced depending on:
 the fragmentation of the molecule
 the occurrence of isotopes.
Fragmentation
When a molecule is bombarded with electrons, a
positive ion is formed. This is called the parent
molecular ion. The parent molecular ion is chemically
unstable (having one less electron than it wants) and
can fragment into more positive ions and uncharged
free radicals. Uncharged free radicals are removed by
vacuum pump.
CH3COOH + e- → CH3COOH + + 2e-
CH3COOH + = parent molecular ion
m/e (CH3COOH +) = 60/1 = 60
CH3+ = fragment ion
m/e (CH3+) = 15/1 = 15
.COOH = uncharged free radical
CH2+ = fragment ion
m/e (CH2+) = 14/1 = 14
CH3COOH + → CH3+ + .COOH
CH3+ → CH2+ + .H
Note: M+ → A+ + .B
M+ → .A + B+
or
1
Isotope effects
Isotopes of ions cause additional peaks to occur due to the difference in mass
(affects m/e ratio!)
E.g. Dichloromethane (CH2Cl2)
 Chlorine has 2 isotopes 35Cl and 37Cl
 3 peaks occur:
- one if there are two 35Cl, m/e = 84
- one if there are two 37Cl, m/e = 88
- one if there is one of each! m/e = 86
Interpreting Mass Spectrums
Peak Height = relative concentration of the ions present.
The highest is called the base peak. This is assigned 100% intensity and all other
peaks are measured against it.
Ion fragment intensity depends on the:
 Energy of the bombarding electrons
 Stability of the ion fragments formed
 Ease with which ions can lose atoms.
2
Combinations
Chromatography and Mass Spectroscopy
 GC – MS
 HPLC – MS
Chromatography separates the components and Mass Spectroscopy identifies them!
As the components elute from the chromatography, they enter the mass
spectrometer and it cycles through the mass / charge ratios to record all the
individual components. These get compared to known samples and identifies them.
This technique is often used in forensic analysis.
Synchrotron
A Synchrotron is basically a collection of mass spectrometers, linked in a circular
manner, covering half a football field.
How it works
 Electrons are accelerated close to the speed of light. This is done by
magnets in a linear accelerator and a booster ring.
 Accelerated electrons are transferred to the storage ring.
 Magnets cause the electrons to travel in circles in the storage ring.
 Electromagnetic radiation is generated by the electrons and this is sent down
beam lines to operate chemical instruments.
Advantages of Synchrotrons
 The electromagnetic radiation produced is exceptionally strong
 A wide energy spectrum is produced – infrared to X-rays
 Precise frequencies can be selected
 The radiation is highly polarised
 Short intense pulses of radiation can be produced.
Uses of Synchrotrons
 Development of anti-influenza drugs
 Chocolate makers find the optimum condition for cocoa crystals
 Heat resistance is measured in ceramic and metal surfaces
 Breathing patterns in beetles can be monitored using X-ray videos.
Very
important
research!
3
Combining Analysis Techniques tells us More!
Read about the Analysis of Ethanol to see what the different techniques find!
pg. 117-119
Which analytical technique to use?
Assessing analytical techniques is based on Chemical factors and Economic factors.
Chemical considerations
 Analyte – what are the samples characteristics? Metal? Coloured? Volatile?
 Sample – amount available, concentration, physical state etc.
Economic considerations
 Time – how fast do you need the results?
 Cost of equipment - $$$$ versus $
 Expertise – does someone know how to operate the machine?
 Quality data – quality costs more.
4